1. Field of the Invention
The present invention relates to a communication system and a packet transmission method, and more particularly, to a communication system comprised of a mobile terminal, a plurality of access routers, each access router forming a cell and configured to perform radio communication with a mobile terminal located in the cell, and a plurality of relay routers connected to a hierarchical structure above the access routers, and a packet transmission method applied in the communication system when a packet from a packet source is transmitted from an upper layer in the communication system toward a target mobile terminal.
The mobile terminal generally represents every portable terminal equipment including cellularphones, PHSs, PDAs, portable personal computers, and so on, and the access routers are routers with the base station function, such as radio interfaces and the like.
2. Related Background Art
It is known in the field of mobile telecommunications that packet loss occurs during handover in which a mobile terminal migrates between access routers. In Cellular IP described in Internet Draft, a relay router (e.g., a cross over router (COR)) temporarily multicasts packets to both an original access router and a new access router during handover, thereby decreasing packet loss during handover.
The multicast handover in Cellular IP will be described below with reference to FIGS. 1A-1D. In the multicast handover, as shown in FIG. 1A, a mobile terminal MT first acquires information about an access router AR2 as a destination of movement on the basis of Link Layer information during communication with an access router AR1 being an original access router before movement. Then, as shown in FIG. 1B, the mobile terminal MT transmits a route update packet for multicast handover via the original access router AR1 to the new access router AR2 whose information was acquired above. Thereafter, a relay router R1 receives the route update message for multicast handover.
Then the relay router R1 receiving the route update message for multicast handover, multicasts packets to the access routers AR1, AR2, as shown in FIG. 1C. Furthermore, as shown in FIG. 1D, the mobile terminal MT, after having moved to under the access router AR2, sends a route update packet to a gateway GW. This results in eliminating the path other than the path via the access router AR2 out of those from communication correspondent 30 to the mobile terminal MT.
In Cellular IP, however, when the relay router multicasts packets to the original access router and to the new access router, no consideration is given to the timing when the relay router sends packets into each of the paths, and this poses the following problem.
In the network of FIG. 2, when a transmission delay of link 1 is greater than that of link 2, the mobile terminal MT might receive packets some ahead of already-received packets after completion of handover, so as to be unable to receive packets between them; therefore, it is necessary to perform retransmission in TCP and the TCP throughput will be lowered.
On the other hand, when the transmission delay of link 1 is not more than that of link 2, the mobile terminal MT receives already-received packets after completion of handover. In TCP all packets are given a series of sequence numbers and, when being unable to receive packets of sequence numbers in the given order, instead of returning the sequence numbers, the receiver sends to the sender an ACK of a last packet of those received in the given order of sequence numbers. Concerning the ACK of this packet, since another ACK must have already been sent upon reception of the packet, the packet sent at this stage is a duplicate ACK (D-ACK).
If the number of duplicate ACKs generated by packets received redundantly becomes three or more, the system goes into the Congestion Avoidance phase called Fast Retransmit and Fast Recovery in TCP Reno, TCP New-Reno, TCP SACK, etc. being the TCP algorithms currently used in general, so as to lower the throughput of TCP in some cases. However, as also described in the Draft of Cellular IP, no packet loss occurs in this case and thus the drop of throughput is likely to be less than that in the aforementioned case where the transmission delay of link 1 is greater than that of link 2.
The present invention has been accomplished in order to solve the above problem, and it is a first object of the invention to calculate a deviation of synchronization between multicast packets from a relay router when received by a mobile terminal, and add a delay to correct for the deviation of synchronization calculated for each relay router by the mobile terminal, to each path in multicasting, thereby preventing occurrence of packet loss.
It is a second object of the invention to implement control adapted to the present handover method on the TCP side of the mobile terminal, thereby preventing the drop of TCP throughput due to duplication of packets.